This invention relates to a method and inhibitor to prevent pitting and general corrosion in aluminum products exposed to aqueous environments, without the use of chromates. Chromate and dichromate salts have been extensively used in the past as corrosion inhibitors for aluminum alloys in aqueous environments. Although these inhibitors are effective, the chromate ion is reported to be toxic and subject to adverse safety and pollution control regulations.
There have been many non-chromate inhibitors developed over the years. In U.S. Pat. No. 2,560,839 dated July 17, 1951, to Ayo et al. is disclosed a corrosion inhibitor for use in conjunction with a detergent for a mechanical washing operation. The corrosion inhibitor prevents the detergent from attacking and tarnishing the metal parts of a washing machine. This patent discloses that about 1 to about 5% by weight of silicates, particularly metasilicate, have been found to be an effective corrosion inhibitor.
In U.S. Pat. No. 3,340,001 dated Sept. 5, 1967 to Thornhill is disclosed a corrosion inhibitor for water used as a cooling medium to protect metals in contact with the water from corrosion thereby. The inhibitor comprises urea, alkali metal cyanate, minor constituents of sodium mercaptobenzthiazole, benztriazole, alkali metal nitrate, alkali metal nitrite, and alkali metal silicate excepting metasilicate.
In U.S. Pat. No. 3,948,792 dated Apr. 6, 1976, to Watsen et al. is disclosed a silicate containing anticorrosion formula for use in automobile and diesel coolant systems. A formula is used which comprises the addition of polymeric dispersants to a silicate containing corrosion inhibitor such as the boron-nitrite type ameliorates.
In British Pat. No. 691,578 published May 13, 1953, is disclosed a corrosion inhibitor for aluminum, aluminum-manganese having a concentration of from 2 to 20% per volume of water and containing a mixture of sodium carbonate and urea and minor constituents of sodium nitrate and sodium silicate.
The prior art in general discloses corrosion inhibitors for use in substantially open loop coolant systems having a relatively short use cycle, such as automotive radiators. For short use purposes high concentrations, i.e., greater than 1%, of inhibitor may be desirable. This has been found not to be the case when an inhibitor is used to prevent corrosion and pitting in a closed-loop system designed for very long term operation, such as a water-cooled fluorescent lighting installation. In such an installation concentrations greater than 1% of inhibitor permit rapid corrosion.